Physics
Newton's laws of motion — quick study summary
Newton's three laws describe the relationship between forces and the motion of objects. They form the foundation of classical mechanics and explain everything from cars stopping at red lights to rockets reaching orbit.
Key points
- 1st law (inertia): an object stays at rest or moves at constant velocity unless acted on by a net force
- 2nd law: F = ma — net force equals mass times acceleration
- 3rd law: for every action there is an equal and opposite reaction
- Forces are vectors — they have magnitude AND direction
- If net force = 0, the object is in equilibrium (still or constant velocity)
In more depth
Resultant force
Newton's laws are about the resultant force, the overall force after combining every force acting on an object. Forces in the same direction add; opposite forces subtract. If the resultant is zero, the object stays at rest or keeps moving at a constant velocity. A non-zero resultant changes its speed or direction.
Using F = ma
The second law links resultant force in newtons, mass in kilograms and acceleration in m/s². Doubling the force doubles the acceleration; doubling the mass halves it. Weight is a force, W = mg, so a 60 kg student weighs about 590 N on Earth, while their mass is 60 kg anywhere.
Action-reaction pairs
Third-law pairs act on different objects, are the same type of force, and are equal in size and opposite in direction. When you push on a wall, the wall pushes back on you. A book's weight and the table's push on it are not a third-law pair; the partner of the book's weight is the book pulling Earth upward.
Common mistakes
- Thinking a moving object needs a constant force to keep moving. With no resultant force, it keeps the same velocity.
- Treating mass and weight as the same thing.
- Saying third-law forces cancel out. They act on different objects, so they cannot cancel.
Practice quiz
Click each question to reveal the answer.
1. A 2 kg object accelerates at 3 m/s². What is the net force on it?
Answer: 6 N
F = ma = 2 × 3 = 6 N.
2. Which Newton's law explains why a passenger jerks forward when a car brakes?
- 1st law (inertia)
- 2nd law (F = ma)
- 3rd law (action-reaction)
- Conservation of momentum
Answer: 1st law (inertia)
3. If you push a wall with 50 N of force, what force does the wall push back with?
Answer: 50 N
Newton's 3rd law — equal and opposite reaction.
Last reviewed: May 2026